The invention relates to radioelectrical antennas and more particularly in such antennas to the electromagnetic window called the radome exposed to atmospheric conditions and crossed by radioelectrical waves.
The problem that is resolved by the invention shall be explained in the context of a radome for an antenna used in a system for landing assistance for aircraft 22 (FIG. 1), a system better known as the MLS or microwave landing system.
A landing assistance system of this kind for a runway 10 comprises (FIG. 1) an azimuth station 12 placed at the end of a track along its axis 18 and an elevation station 14 placed slightly after the beginning of the runway to the side.
The azimuth station 12 sends out radioelectrical waves in a radiation pattern in the vertical plane that shifts in azimuth between -40.degree. and +40.degree. with respect to the vertical plane 16 containing the axis 18 of the runway 10. This shift is achieved in proportion to time in steps of five-thousandths of a degree. This is shown by the drawing of FIG. 2 for a small number of steps;
The elevation station 14 too sends out radioelectrical waves but does so according to a radiation pattern that shifts in elevation angle from 0.9.degree. to 15.degree. in proportion to time in steps of five-thousandths of a degree. This is shown in the drawing of FIG. 3 for a small number of steps. One of these radiation patterns in elevation contains the axis 20 of descent of the aircraft 22 towards the landing point on the runway 10. The values of the steps that have been indicated here above by way of an example show the precision with which the positions of the different planes of radiation must be known, in such a way that all causes of error must be eliminated.
One of these causes of error is rain which streams down the radome of the antenna of the elevation station 14. For, an elevation station of this kind is formed by an emission antenna 30 (FIG. 4) positioned on a pedestal 32. This antenna is placed in a shelter 34 that contains the emitter and its control elements which are not shown. The lateral side of the shelter 34 facing the antenna 30 has an aperture 36. In this aperture 36, there is placed a plate 38 that is transparent to the radioelectrical waves emitted by the antenna. This plate is called a radome. For example, the dimensions of such a radome for an elevation station are 1.20 m in width and 4 m in height.
With a radome height of this kind it will be understood that, in the event of heavy rain (storms), the water that streams from the top to the bottom of the radome has a thickness that increases as and when the base of the radome is approached. This modifies the overall characteristics of the radome. This sheet of water, which is thicker at the bottom than at the top, constitutes a sort of prism or electromagnetic lens that has two types of effects on the radioelectrical waves that cross it:
the introduction between the top and bottom of the radome of a phase gradient achieving an undesirable transfer function applied to the phase relationship of the antenna; PA1 the homogeneous mismatching and attenuation of the radome creating standing waves between the radome and the antenna and modifying the relationship of distribution of the antenna. PA1 the aiming precision of the beam, PA1 the revival of minor lobes in the antenna pattern, PA1 the gain of the antenna.
These disturbances affect the performance characteristics of the antenna chiefly with respect to three parameters: